Abstract: In this study, a-C:H film was prepared by midfrequency reactive magnetron sputtering system, and a ball-on-disk reciprocating tribometer was used to investigate the friction behaviors of a-C:H film in air and dry nitrogen (N₂) atmospheres. The influence of structural changes of wear tracks and transfer films on Al₂O₃ counterpart balls between sliding interfaces on friction behaviors were discussed. The results showed that the a-C:H film had lower friction coefficient and longer wear life in nitrogen than in air. The microstructure analysis showed that the transfer film reduces friction. In dry nitrogen, a stable carbon transfer film with typical DLC characteristic formed on Al₂O₃ counterpart balls, which further improved the friction performance. In addition, in dry nitrogen, the structure of the wear track and the transfer film tended to be “graphitized”. Therefore, it is the combined action of the above two that resulted in lower friction and wear of a-C:H film in N₂ atmosphere than in air.